This invention relates to mobile heavy-duty equipment and more particularly to an improved off-road, heavy-duty haulage vehicle, hereinafter referred to as a "vehicle".
Historically, off-road, heavy-duty work vehicles such as trucks have included front wheels, motorized rear wheels, a chassis or frame, front and rear suspension systems connecting the wheels to the frame, a body including a motor compartment and cab connected to chassis, a diesel electric traction system attached to the frame inside the motor compartment and connected to the motors of the motorized rear wheels, vehicle operating controls mounted within the cab for an operator, and a bed attached to the frame for transporting a load.
In the past, diesel electric traction systems (having a diesel engine driving a generator or rectified alternator) have used direct current series motors as traction motors. Control of voltage, amperage, and load on the diesel engine has been accomplished by regulating the generator field. Traction motors were used also to retard the truck and were excited by the generator; excitation was controlled by the generator field.
More recently, two distinct separately excited systems have been used. One of these uses tertiary windings placed on the armature of the rectified alternator to supply excitation both for the motor fields and for the alternator field. The second maintains excitation of the alternator field and taps the main windings for field supply, both for the alternator field and for the motor field. Excitation levels are controlled by SCR bridges.
In the first system, using tertiary windings as a source of excitation, the alternator field is still used to limit volts and amps and regulate power to the motor armatures. Further, the direct motor field is controlled by an alternating current rectifier bridge. The tertiary windings are placed on the armature of the alternator in such a way that they are coupled magnetically with the main armature current as well as air gap flux, thus, creating the ability to generate currents at low motor speeds when counter emf of the motors is low, and air gap flux is very low. Motor field current in these systems is regulated proportionally to armature current with added provision for weakening the motor field at high motor speeds.
The disadvantages of this system are as follows: as armature current tends to change rapidly compared to motor field which is highly inductive, poor commutation and brush life result from the motor field always following the motor armature current; initial response of the system is slow because armature current must buildup before any field supply is available; and large contactors are still required to interrupt the power circuit, and response time is slowed by switching times of the contactors.
In the second system, a phase angle controlled SCR armature converter is used to control armature current and SCR bridges are used to control field levels. This system has typically been used with constant speed engines.
The disadvantages of systems tapping the main winding for excitation are as follows: excitation currents in the alternator are very high at low motor speeds when high motor armature amps are required; the leakage reactances and armature reaction of the armature winding create high excitation current demand at high line ampere levels in the alternator; the necessity to maintain high line voltage in order to supply motor field amperes adds significant excitation requirement and increases thermal losses in the field of the alternator; the high line voltages at low motor speeds cause the armature converter to fire for only a small portion of the total cycle and thus causes inefficient use of the SCRs; and the small angle of conduction at high currents in the armature circuit causes high rms currents in the armature of the motor which have a negative impact on motor thermal capacity and commutation.
The present off-road, heavy-duty vehicles also suffer from excessive maintenance and repair requirements, and from response times which increase both operating costs and safety risks.
The advantages of the present invention over the prior art devices are that a programmable controller regulates all power control functions including the matching of engine capability to power and speed requirements of the vehicle; the inputs to the controller are dual to actuate events based on the highest speed wheel and the controller provides a signal for controlling speedometer indications and making recorder measurements.
Thus, in response to operator signals, main contactors set up appropriate circuits between the alternator and the drive motors--before excitation causes generation of power. Forward or Reverse propulsion circuits result from the corresponding position of the operator's selector lever and acceleration and speed is maintained by a foot pedal. Excitation of the main alternator is modulated by the control system in response to the foot pedal operation to provide power proportional to engine speed. The load increases smoothly as engine speed increases to insure maximum fuel economy and engine efficiency. Engine bogging from overloading is prevented and at full throttle, the control systems maintains full governed engine speed and constant horsepower over a wide range of vehicle speeds.
As vehicle speed increases, the control system triggers a series of automatic events. For example, anti-reversal occurs at vehicle speeds over three MPH. Further, in the retard mode, the wheel drive motors act as generators whose power is fed back to the A.C. generator to slow the diesel engine to provide excellent retarding effort at low speed without the need for large expensive contactors.
Another advantage example, is the complete monitoring of the system by the controller which signals faults and impending problems, shuts down the power plant when problems are indicated which will result in serious damage to the system, and records the results of the monitoring operation for use in troubleshooting and regular maintenance.